The present invention relates to a radioactive implant, such as a seed for use in brachytherapy, and to a method for imparting radioactive properties to an implant substrate.
Radiation therapy has long been used to treat cancer and other diseases of the body. It is understood that the targeted application of radiation destroys the cells in the targeted area, such as rapidly multiplying cells, i.e., cancerous cells, thereby providing a disease therapy. One form of radiation therapy is brachytherapy, which generally provides a targeted application of a relatively short half-life radiation source. The radiation source, typically called a brachytherapy seed or implant, is implanted in a specific area to be treated, such as directly at an identified tumor location. Depending on factors such as the size and shape of the tumor and the type of treatment desired, the radioactive source is kept either permanently in the body or removed after a period of time.
Various forms of implants are known for use in brachytherapy, such as radium needles, ribbons and capsules. Multi-layered implants are also known in which a radioactive layer is coated upon a substrate. Examples of such multi-layered implants are discussed in Good, U.S. Pat. No. 5,342,283. In such multi-layered implants it is known to provide a substrate, such as a microsphere, ribbon or fabric and then to form a radioactive layer on the substrate using various methods such as plating techniques including sputtering, cathode arc plasma deposition, laser ablation of the target material in the presence of a radioactive gas or reaction of an excited radionuclide gas with a target material. A disadvantage of such techniques is difficulty in achieving a combination of high ion fluence at a relatively high ion energy to achieve a high concentration of a radioactive isotope such as xenon or another radioactive isotope of an inert gas, such as argon, krypton, or radon. Other known techniques involve very high temperatures above the melting points of the materials being used to manufacture the implants.
Ion implantation is a known methodology for introducing dopant atoms into a silicon substrate, such as in semiconductor chip manufacturing. Generally speaking, ions are directed with varying degrees of acceleration towards the silicon semiconductor substrate. The ions are accelerated by a high voltage differential created between the ion source and the semiconductor substrate (or a cathode adjacent the substrate). The ions are introduced into the substrate by virtue of the momentum of the ions causing them to become embedded within the crystalline lattice of the semiconductor substrate. The implanted ions are understood to cause interference with the lattice structure by being implanted in a random fashion. Annealing is typically used to restore the lattice structure and allow the implanted atoms to migrate to occupy lattice sites. Examples of ion implantation techniques and apparatus can be found, for example, in commonly assigned U.S. Pat. Nos. 4,421,988, 4,433,247, and 5,711,812.
For medical applications, inert gases provide desirable sources of radioactive ions. However, known methods and apparatus for creating radioactive medical implants do not provide an economical or efficient technique for implanting an inert gas in a substrate. Although sputtering has been proposed, the sputtering process works at lower energies and does not result in an efficient incorporation of gas ions into the substrate.
From the above, it is apparent that there is a need for an improved system to manufacture radioactive medical implants such as stents or brachytherapy seeds. It is accordingly an objective of the present invention to provide a system and method using ion implantation for effectively making stents, brachytherapy seeds or other medical implants having a radioactive isotope implanted in a substrate and preferably providing effective control of the radioactive dosage imparted.
It is a further object of the present invention to provide a system and method of achieving high concentrations of a radioactive isotope of an inert gas, such as xenon-133 in a medical implant.
In order to achieve a medical implant that does not readily degrade in use, it is a further objective of the present invention to provide a system and method for implanting a radioactive isotope below the surface of a medical implant substrate.
It is another object of the invention to provide a medical implant having a determined amount of a radioactive ion implanted in its outer crystalline lattice.
It is a further object of the present invention to improve the dosage and uniformity of the radioactive layer used in radioactive implant production.
It is another object of the present invention to provide an improved method of making radioactive implants from a medical substrate, such as silicon, having a thermally stable radioactive layer and a high concentration of ions.
A still further object of the present invention is to provide a method of preparing a radioactive implant with the ability to implant ions in a closed vacuum system, which allows for efficient disposal of any excess radionuclide into a storage cylinder for disposal after sufficient decay has occurred.
Another object of the present invention is to provide a method of making a radioactive implant and for manufacturing a radioactive implant, allowing for increased accuracy in determination of the radioactive dose control, and achieving a greater degree of dosage uniformity.
The present invention alleviates to a great extent the disadvantages of the known structures and systems for manufacturing radioactive implants and achieves the objects noted above, by providing an apparatus and method of doping a silicon based substrate material with a radioactive ion, thereby implanting the ion within the substrate material. An ion implantation doping technique is used to bombard the silicon based substrate with a radioactive ion, preferably xenon-133, or other inert gas such as argon, krypton or radon. The radioactive dosage is controlled by controlling various bombardment parameters including time, voltage and the amount of the surface area of the substrate material targeted. A preferred implant has a radioactive xenon ion implanted in a silicon matrix.
In one embodiment, a silicon substrate is a pre-formed implant such as a stent or other shape, or alternatively may be a substrate material that is suitable for later formation into a useful device. The substrate material preferably is silicon and may be of any form, such as amorphous or crystalline. In one example, the substrate material is preformed into a medical stent. In another embodiment, a thin layer of silicon may be formed over another appropriate base material. The application of the present invention is not limited to particular shapes or sizes of devices irradiated.
Any technique of ion implantation may be used to form the radioactive layer. In one embodiment, a closed vacuum system is used, in which one takes the steps of pumping a leak-tight vacuum chamber to a low base pressure; introducing xenon-133 gas to a few millitorr; igniting a plasma; creating a sufficient voltage differential to accelerate the ions to the target silicon substrate material; extinguishing the plasma when the desired dose is achieved and pumping out the excess xenon-133 into a storage cylinder for disposal after sufficient decay has occurred. A doped radioactive region is formed on the surface of the implant at various energy levels, such as between 1 and 200 keV, and corresponding ion fluences between 6.5xc3x971015 and 5.7xc3x971016/cm2, although any ion implantation energy and ion fluence that can achieve a sufficient amount of doping may be used. These preferred ion implantation energies and ion fluences can achieve an average concentration of 10 atomic percent xenon-133 at the surface of the substrate material. The implant may be post-annealed at high temperature to provide a thermally stable xenon-133 atom-silicon atom couple. Typically, the radioactive ions are implanted on all the exposed surfaces of the substrate, although specific regions of the substrate may be selected, such as by using masking.
One advantage of the present invention is the ability to independently control ion-fluence, ion-impingement rates and the ion-energy in the ion implantation process, thus providing control of the concentration of radionuclide into the substrate or target. Another advantage of the invention is the ability to implant ions in a closed vacuum system in a controlled way, thereby minimizing handling problems associated with disposal of radioactive materials.
It is a further advantage of the present invention that a system and method of achieving high concentrations of a radioactive isotope of an inert gas, such as xenon-133 in a medical implant is provided.
Another advantage of the present invention is that a medical implant is achieved that does not readily degrade in use because the radioactive isotope is implanted below the surface of a medical implant substrate.